"Covered Roll" vs. "Uncovered Roll" Treater Stations

By Pillar Technologies

There are two different types of treater stations, one designed to treat conductive substrates, now known as bare roll, and the other designed to treat non-conductive substrates, known as covered roll. The primary difference between these designs is the location of the dielectric material, and the method used to cool the dielectric.

Conductive or metallized substrates require the dielectric to be on the "hot" electrode, so the high voltage won't arc directly to the substrate. Early designs used a series of driven covered rolls mounted around a bare grounded drum. The roll's rotation was intended to circulate the surrounding air, keeping the dielectric cool. When the bare roll station was introduced in 1979, it utilized stationary dielectric covered electrodes, such as metal filled ceramic tubes, metal filled quartz tubes, or ceramic covered metal shoes, which are cooled by increased volumes of exhaust air that is extracted over the electrodes.

Non-conductive substrates are treated with "covered roll" systems which incorporate a "hot" metal and a dielectric covered grounded drum. Like the bare roll design, the roll's rotation is used to keep the dielectric cool.

Unlike the covered roll stations, the bare roll design can be used on either conductive or nonconductive substrates. Although this flexibility is advantageous to the flexo printer since both types of substrates are frequently used, the use of conductive designs on nonconductive webs can result in reduced efficiency and treat level. For example, on one BOPP film, the amount of power needed to obtain 39 dynes on a bare roll system, would achieve 48 dynes on a covered roll station. This is not to say that this type of difference will occur on all materials, but it is common to expect higher treat levels on nonconductive materials with covered roll systems.

Since most bare roll style treaters are designed for fixed treat widths, lane treating applications are often restricted to covered roll style stations. The use of bare roll style treaters on non-metallized substrates became popular in converting operations during the early 1980's due to the increased reliability of the ceramic dielectric. The use of ceramic coverings on covered roll designs has eliminated this advantage.

The proliferation of waterbased inks further expanded the use of bare roll treaters since they were capable of treating both types of substrates. However, with the increased use of hard to treat materials, such as BOPP and high slip LLDPE, and the need for higher treat levels at faster line speeds, the need to optimize treater performance has become important.

The flexo printer tends to work with a variety of substrates. If conductive materials are intended to be used, a "bare roll" style station is necessary. This style would be capable of treating the easier to treat nonconductive substrates as well. However, if the hard to treat nonconductive materials are to be printed on, the printer is better off with a "covered roll" design. The ideal solution for these situations was the introduction of the universal systems which can easily be changed from bare roll to covered roll - - this way the converter can have the best of both worlds.

Convertible designs have been around for a long time, however, they have recently become more user friendly. One type of system enables the printer to change from conductive to nonconductive by simply rethreading the web. Other designs involve changing electrodes. One of these designs has both conductive and nonconductive type electrodes mounted on a support bar, which enables the operator to pivot the desired electrode into position. Another design utilizes electrode magazines. The operator changes from one style to the next by plugging into the different magazines.

In the past, the debate between bare roll and covered roll was something every flexo printer had to consider. The introduction of convertible style designs have eased this decision although they must warrant the need. The variety of treater station designs enable flexo printers to use the style treater that is best for the specific job they're running.